The delivery of a cool, dry air stream is necessary for a variety of applications ranging from industrial processes (e.g. plastics, food processing), to comfort control of large indoor spaces, to clean room environment control. Air handling chambers are designed to house the appurtenances necessary for the treatment of such air flow streams. The chambers are designed to accommodate a variety of components, depending on the application (e.g. cooling coils, desiccant wheels, and filtration systems).
The temperature within an operating air handling chamber is often substantially below the temperature surrounding the chamber. Such chambers are often deployed in high humidity environments. For example, outdoor or roof mounted chambers are routinely exposed to high temperature, high humidity ambient conditions associated with summer time operation. Indoor units are often installed within a high humidity environment associated with the process that requires air handling.
Conventional air handling chambers utilize a modular panel design. The walls of the chamber are constructed from pre-formed panels that mate with each other along jointed seams. The panels typically have a hard (often metallic) shell that is filled with a thermal insulation material. Some modular panel designs feature edges that are enclosed with the shell material, so that the mating edges of abutting panels have a stiff interface suitable for the insertion of a sealing material. The shell, typically constructed from a higher thermal conductivity material than the insulation material within, thermally bridges the thickness of the panel, creating a zone of lower temperature on the shell exterior along the seam of the joint. Condensation can form and accumulate when the temperature of these zones fall below the dew point temperature of the surrounding air.
Other designs leave the insulation exposed on the panel edges, the insulating material of one panel being formed to mate directly with the insulation of an adjoining panel. Such designs are more difficult to seal with interstitial materials at the joints and are prone to leakage of the cooler interior air because insulation materials tend to be of lower density and are less resistant to wear. Leakage through the joints effectively cools the outer surfaces of the panels near the seams, which also leads to the formation and accumulation of condensation on the exterior shell.
Conventional air handling chambers also utilize a base design that is prone to the formation of external condensation. Some chambers house heavy components, such as high capacity compressors or large banks of air-to-fluid heat exchangers. For the sake of rigidity, standard base structures form a thermal bridge between the chamber interior and the exterior of the base.
The food processing industry is particularly sensitive to condensation or “sweating” on the exterior of air handling equipment. Accumulation of condensation leads to the formation of droplets that can fall into food products or otherwise contaminate sanitized areas. Even outdoor units can cause contamination of food processing areas. For example, a roof-mounted unit typically has ductwork that extends from the bottom of the chamber and into the building through the roof. Condensation that forms on the exterior of the walls and base of the chamber can flow downward, attach to exterior of the ducting and make its way into the food processing area, thereby posing a contamination risk. The Food and Drug Administration has recognized the health risks associated with condensation in food processing facilities, and has promulgated rules and guidelines regarding condensation on air handling enclosures. See, e.g., 9 CFR Part 416, “Sanitation Requirements for Official Meat and Poultry Establishments, Final Rule,” 2000.
Heat flux through a solid medium, expressed in Watts per square meter, is directly proportional to the thermal conductivity of the medium (hereinafter referred to as k) and inversely proportional to the thermal path length (hereinafter referred to as L). That is, heat flux is proportional to the ratio k/L. In the case of a planar wall such as utilized in a thermal isolation chamber, the thermal path length L is dominated by the thickness of the insulation between the inner and outer wall assembly. A thicker wall enables the use of a higher conductivity material, whereas a thin wall requires the use of a lower conductivity material to maintain the exterior temperatures above the dew point temperature.
Generally, the thermal conductivity of so-called “thermal insulation” or “thermal insulative” materials can be of any magnitude, provided the available thermal path length L is long enough (i.e. the wall is thick enough) to maintain the exterior temperatures above the dew point temperature.
There exists a need for an air handling chamber design that minimizes or avoids the formation of condensation on exterior surfaces, yet is readily adapted to the construction of chambers of various sizes.